Martensitic steel for high temperature application



United States Patent MARTENSITIC STEEL FOR HIGH TEMPERATURE APPLICATION Remus A. Lula, Brackenridge, Pa., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania No Drawing. Application May 10, 1956 Serial No. 583,949

4 Claims. (Cl. 148-31) This invention relates to improvements in stainless steels and in particular to martensitic stainless steels for use at elevated temperatures.

Heretofore, there have been many steels and alloys developed for use at elevated temperatures. Foremost among the alloys which have been developed heretofore are those known to the trade as super alloys. However, the super alloys contain an over abundance of costly and highly strategic alloying elements; therefore, the use of such super alloys has been restricted by theirv relativelyhigh cost and the scarcity of the strategic elements.

Modern trends have witnessed the use of higher and higher temperatures especially in applications such as gas turbines and steam turbines. The metal industry has therefore attempted, because of the high cost of the super alloys, to develop iron base alloys having the properties of good strength both at room and elevated temperatures up to 1200 F., good internal damping capacity, resistance to erosion and corrosion by condensation in steam turbines and by the combustion prodducts in the gas turbines together with resistance to stress corrosion and low linear expansion characteristics when the alloys are used at elevated temperatures up to 1200" F. While austenitic type stainless steels have been used in the past because of their ease of weldability and low loss of strength whensubjected to overheating together with less possibility of heterogeneity due to segregation, the austenitic type stainless steels have not proved too satisfactory in operation because of their low proof and maximum stress and expansion characteristics. The metal industry has thus adverted to the use of ferritic and martensitic types of stainless steels since such steels possess higher proof and maximum stress at lower temperatures, provide greater ease in machining and have a lower linear expansion which thereby results in lower thermal stress, and from an economic standpoint, effects savings in the production and fabricating costs of the resultant product.

An object of this invention is to provide a martensitic stainless steel for use at elevated temperatures.

Another object of this invention is to provide a martensitic stainless steel having excellent stress-rupture properties, high proof and maximum stresse together A more specific object of this invention is to provide a martensitic stainless steel having a balanced composl-- tion Within definite limits comprising carbon, manganese, silicon, chromium, tungsten, molybdenum, vanadium, nitrogen, nickel, columbium and the balance iron with the 2,853,410 Patented Sept. 23, 1958 prises a balance of the austenitizing elements against the ferritizing elements in such a manner that upon heat treatment at temperatures to be described hereinafter,

.a maximum of 10% delta ferrite can be formed and upon subsequent cooling and transformation, a minimum of 90% martensite is formed. While up to 10% delta ferrite can be tolerated in some applications where operating conditions are not too severe, it is preferred to maintain a completely martensitic structure in order o obtain maximum properties in the alloy. The alloy of this invention comprises generally the elements of carbon, manganese, silicon, chromium, tungsten, molybdenum, vanadium, nitrogen, nickel, columbium and the balance iron with incidental impurities, each of foregoing elements being present within a given range.

Reference may be had to Table I illustrating the general range and the preferred range within which the elements of this alloy may be varied.

Table 1 Chemical composition-percent by wt.

Element General Optimum Range Range 0-15-0. 35 0. 20-0. 27 0. 50-2. 00 0. -1. 25 Up to 0.75 0 20-0. 40 10.50-13.50 ll 25-11. 75 0 30-0. 75 0 35-0. 50 2 00-3. 50 2 75-3. 25 0 40-0. 75 0 40-0. 60 0 025-0. 10 0. 05-0. 09 0 30-2. 50 0. 40-1. 00 0 20-1. 50 0 20-0. 50 B Ba].

The elements specified are selected to perform definite functions and are maintained within the ranges given for reasons given hereafter.

Carbon is employed in the alloy for imparting strength and hardness thereto as well as to render the alloy susceptible to heat treatment, a minimum of 0.15%

being needed in order to produce at least martensite in the structure when the alloy is in the heat treated condition. This minimum must also be maintained in order to insure that under all conditions the martensite formed upon quenching from the heat treating temperatures is the continuous phase. A maximum of 0.35% carbon must not be exceeded as such larger carbon contents detritnentally affects the alloys resistance to corrosion and oxidation. Further, an excess of carbon in the alloy, will form an excess of carbide in the microstructure With the result that the alloy will have an abnormally high hardness which detracts from the inachineability and formability of the alloy. Carbon within the range given aids in suppressing the formation of delta ferrite when heat treated as will be described hereinafter by expanding the temperature range inwhich austenite i formed.

Manganese is an essential element within the range stated in Table I in that it contributes to the balancing of the ferritizing elements in addition to providing for proper deoxidation and contributing to the ease of rolling. The upper limit of 2.0% must not be exceeded for any increase over this amount will decrease the characteristic of resistance to oxidation in the resulting alloy. Manganese like carbon, is an austenitic forming element, which also contributes to the suppression of delta ferrite.

Silicon is an essential element which must be maintained at not more than 0.50%, in order to minimize its ferritizing efiect which upon heat treatment contributes to formation of delta ferrite.

Chromium, within the range set forth is the primary element which provides the resistance to oxidation and corrosion of the alloy of thi invention. A minimum of 10.50% must be maintained in order to insure adequate resistance to oxidation and corrosion at elevated temperatures. While it is known that increasing chromium contents above 10.50% provides greater resistance to oxidation and corrosion, however, since chromium is a strong ferritizing element, the maximum of 13.5% must not be exceeded in this alloy in order to control the formation of delta ferrite and insure the formation of sufiicient (90% minimum) austenite upon heat treatment.

Vanadium contributes to the hardness of the solid solution of the alloy of this invention. At least 0.30% is needed for the proper contribution of vanadium to the high temperature properties. On the other hand, increasing the vanadium over 0.75% shows no significant or beneficial effect upon this alloy of this invention.

Tungsten is present in an amount ranging between 2.0% and 3.5%. Since tungsten is a strong carbide former, its effect upon the alloy is to contribute to the balance between the austenite and the ferrite. It has been found that the maximum of 3.5% cannot be exceeded as otherwise the requisite amount of austenite will not be formed in the alloy upon heat treatment with the result that when the steel transformed, less than the 90% minimum martensite will be obtained. However, the tungsten content must be maintained at a minimum of 2.0% be cause it is the principal element for providing the requisite high temperature properties and it also contributes to the hardenability of this alloy.

Molybdenum is employed in this alloy for its contribution to the high temperature properties. At least 0.40% is needed for the hardening of the solid solution. While increasing the molybdenum content will increase the hard ness and the strength of the solid solution of this alloy, 0.75% has been found to be the maximum over which no substantial improvement in the solid solution strength or hardness has been observed. In this particular type of alloy and especially for its intended use, an increase of the molybdenum content of over 0.75% may have the adverse effect of making the alloy susceptible to catastrophic oxidation.

It is to be noted also that molybdenum cannot be substituted for tungsten and vice versa in the alloy of this invention since it has been found that molybdenum is a stronger ferritizing element than tungsten with the result that where molybdenum is present in amounts greater than 0.75% as a substitute for a portion of the tungsten of the range given hereinbefore, delta ferrite in amounts greater than the permissible maximum of is formed. Moreover, higher molybdenum contents over the range given in Table I contribute toward the formation of sigma phase and 885 F. embrittlement thereby seriously affecting the properties when used at elevated temperatures by decreasing the impact strength and considerably reducing the bend angle ductility. It is also believed that large contents of molybdenurmhave an influence on the formation of chi phase which detrimentally affects the mechanical properties by embrittling the alloys which contain high molybdenum contents.

Nitrogen is employed as an austenite forming element and as such is used to balance the ferritizing elements. It also contributes to the hardness and strength of the alloy but must be limited however, to 0.10% maximum because an increase over this amount will present difliculties in casting.

Nickel in the range given is used for the purpose of balancing the ferritizing elements in the alloy and has the effect of decreasing the possibility of segregation and of broadening the temperature range in which austenite may be formed thereby suppressing the formation of delta ferrite at elevated temperatures.

Columbium is employed in this alloy in order to increase the solid solution strength thereof, it having been found that about 0.20% is the minimum below which no appreciable increase in strength is noted. On the other hand, the columbium content must be limited to 1.5% maximum because it has a strong carbide forming tendency which contributes to the formation of delta ferrite. The balance of the alloy comprises iron with incidental impurities such as 0.04% maximum each of sulphur and phosphorus, 0.5% maximum copper and incidental amounts of other elements normally found in the production of stainless steel.

It is to be noted that in'Table I, each element in the general range may be varied within the limits set forth therein to obtain a steel having a minimum of martensite in the heat treated condition. However, it is also possible to select a combination of elements such that the steel would posses more than 10% delta ferrite and still be within the general range. It is therefore ap parent that when selecting the steel within the general range of elements set forth in Table I, the austenite and ferrite forming elements must be balanced in such a manner to insure a minimum of 90% martensite in the steel in the heat treated condition. On the other hand, the range of elements as set forth in the optimum range is such that regardless of the combinations of ferrite and austenite forming elements selected, the steel in the heat treated condition will always have at least 90% minimum martensite.

Reference may be had to Table II which illustrates'the composition of Alloy DE34 which is outside of the general range, Alloys DL-28 and DK-23 which are within 3 the general range, the nominal composition of a ferritic alloy known to the trade today and designated as Alloy A, and Alloy B whose nominal composition is that of the well-known 18-8 austenitic type stainless steels.

Table II DE-34 DL-28 DK-23 0. 16 0. 22 0. 22 0. 72 1. 03 0. 90 0t 51 0. 36 0 30 12.27 11.58 11. 67 0. 35 0. 40 0.43 2. 87 3. 11 3. 40 0. 48 0. 53 0. 50 0. 55 O. 45 0. 43 0. 25 0. 98 Ba]. Bal. Bal.

(n) Nominal composition.

of the alloy of this invention over both the well-known ferritic type of stainless of Alloy A and austenitic type stainless steel of Alloy B as well as the effect of columbium on the basic alloy of this invention, reference may.

be had to Table III which contains the stress rupture properties for each of the alloys whose composition is forth in Table II.

set I Table III 1,100 F. Stress for Rupture, Stress for Rupture, p. s. i. in-

p. s. 1. m- Heat No. Heat Treatment 100 1,000 10,000 100 1,000 10,000 Hrs. Hrs. Hrs. Hrs. Hrs. Hrs.

DIE-34"--. 1,900 F. 1 Hr., O. Q.,

2 Hr. 1,200 F 60,000 44,000 33,000 19, 000 DL-28 --d 68,000 49,000 34,000 85,000 22,000 14,000 DK-23 do 60, 000 48, 000 40,000 38,000 25,000 19,000 Alloy A do 47,000 37, 000 23,000 24, 000 16,000 Alloy B 2,012 F. 1 Hr., W

2 Hr. 1,200 F 40,000 25,000 17,000 23,000 17,000

1 Extrapolated.

By inspection of the test results recorded in Table 111 it is seen that both at 1100 F. and 1200 F. alloy A possesses far superior stress rupture properties than Alloy B. However, the stress rupture properties of both Alloy A and Alloy B are far inferior to those of Alloy DE-34 both at 1100 F. and 1200 F. It is readily seen that at 1100 F. a stress of 40,000 p. s. i. is required to produce rupture in 100 hours for Alloy B, whereas a stress of 47,000 p. s. i. is needed to produce rupture in Alloy A. When compared to DE-34 it is seen that at 1100 F. it requires a stress of 60,000 p. s. i. to produce rupture in 100 hours. As the time is increased from 100 hours to 1,000 hours the stress required to produce rupture in Alloy DE-34 is far greater than the stress required for either Alloy A or Alloy B. The same is true when the stresses necessary to produce rupture in 10,000 hours at 1100 F. are compared. However, with the addition of columbium to the basic alloy of Alloy DE-34, for example, the addition of 0.25% columbium as in Alloy DL-28 or the addition of about 0.98% columbium as in Alloy DK-23, it is seen that the latter two alloys have excellent stress rupture characteristics in that substantially higher stresses are required to produce rupture when tested at 1100 F. for 100 hours, 1,000 hours and 10,000 hours, respectively. While the addition of higher' amounts of columbium, that is, increasing the columbium from 0.25% to 0.98% shows no substantial increase in the stress required to produce rupture in 100 hours and 1,000 hours, a substantial increase is noted at longer time periods when tested at 1100 F. On the other hand, if higher temperatures are employed, the higher amounts of columbium are preferred because as illustrated in Table III, the alloy having such higher amounts of columbium has better stress rupture properties at 1200 F., in that greater stresses are required for the time periods of 100, 1,000 and 10,000 hours than the stresses required when lower amounts of columbium are used. Thus it is apparent from the test results recorded in Table III that columbium substantially contributes to the stress rupture properties of the alloy of this invention.

In practice, in order to develop the maximum properties, the alloy of this invention is subjected to an austenitizing treatment followed by a tempering treatment. The austenitizing treatment is given at a temperature within the range of between 1800 F. and 2150 F. for a time period ranging between minutes and 2 hours, depending upon the thickness of metal which is to be heat treated. In other words, the alloy should be held within this temperature range for a period of time sufficient to develop the required temperature in order to form at least 90% austenite before the alloy is quenched therefrom. The quenching from the austenitizing temperature must be accomplished with sufficient severity to transform the austenite to produce at least 90% martensite within the alloy in the as-quenched condition. In practice it has been found that an oil quench has been suflicient to accomplish this purpose. It will be appreciated that where the section of the metal being heat treated is sufficiently thin, an air quench may suflice to produce the required minimum of martensite. The quenching is followed by a tempering treatment at a temperature in the range between 1000 F. and 1350 F. for a time period ranging between one and sixteen hours. The tempering treatment has the effect of increasing the ductility and impact strength without sacrificing the strength of the alloy.

As indicated hereinbefore there are no special processing or techniques required to produce the alloy of this invention which is superior to the presently known alloys of the iron base composition. Standard mill processing techniques are employed throughout the manufacture and process of this alloy with the presently used commercial production equipment. These alloys can be readily produced by any one skilled in the art.

I claim:

1. A martensitic stainless steel for use at elevated temperatures consisting of from about 0.15% to 0.35% carbon, from about 0.50% to 2.0% manganese, up to 0.75% silicon, from about 10.50% to 13.5% chromium, from about 0.30% to 0.75% vanadium, from about 2.0% to 3.5% tungsten, from about 0.40% to 0.75% molybdenum, from about 0.025% to 0.10% nitrogen, from about 0.3% to 2.5% nickel, from about 0.20% to 1.5% columbium and the balance iron with not more than 1.5% of incidental impurities.

2. A martensitic stainless steel for use at elevated temperatures consisting of from 0.20% to 0.27% carbon, from 0.75 to 1.25% manganese, from 0.20% to 0.40% silicon, from 11.25% to 11.75% chromium, from 0.35 to 0.50% vanadium, from 2.75% to 3.25% tungsten, from 0.40% to 0.60% molybdenum, from 0.05% to 0.09% nitrogen, from 0.90% to 1.0% nickel, from 0.20% to 0.5% columbium and the balance iron with incidental impurities.

3. A martensitic stainless steel for use at elevated temperatures consisting of about 0.22% carbon, about 1.03% manganese, about 0.36% silicon, about 11.58% chromium, about 0.40% nickel, about 3.11% tungsten, about 0.53% molybdenum, about 0.95% vanadium, about 0.25% columbium, and the balance iron with incidental impurities.

4. A martensitic stainless steel for use at elevated temperatures consisting of about 0.20% carbon, about 0.90% manganese, about 0.30% silicon, about 11.67% chromium, about 0.43% nickel, about 3.40% tungsten, about 0.50% molybdenum, about 0.43% vanadium, about 0.98% columbium and the balance iron with incidental impurities.

References Cited in the file of this patent UNITED STATES PATENTS 2,572,191 Payson Oct. 23, 1951 2,590,835 Kirkby et al. Apr. 1, 1952 2,745,740 Jackson May 15, 1956 

1. A MARTENSITIC STAINLESS STEEL FOR USE AT ELEVATED TEMPERATURES CONSISTING OF FROM ABOUT 0.15% TO 0.35% CARBON, FROM ABOUT 0.50% TO 2.0% MAGANESE, UP TO 0.75% SILICON, FROM ABOUT 10.50% TO 13.5% CHROMIUM, FROM ABOUT 0.30% TO 0.75% VANADIUM, FROM ABOUT 2.0% TO 3.5% TUGSTEN, FROM ABOUT 0.40% TO 0.75% MOLYBDENUM, FROM ABOUT 0.025% TO 0.10% NITROGEN, FROM ABOUT 0.3% TO 2.5% NICKEL, FROM ABOUT 0.2% TO 1.5% COLUMBIUM AND THE BALANCE IRON WITH BOT MORE THAN 1.5% OF INCIDENTAL IMPURITIES. 